Since the first report of high efficiency organic light-emitting devices (OLEDs), OLEDs have attracted extensive interest because of their enormous potential for applications in full-color flat panel displays. Device performance of OLEDs strongly depends on a balanced injection of holes and electrons into the emitting layer from the anode and the cathode. It is generally believed that lowering the injection barrier between the electrodes and the organic medium will facilitate effective carrier injection. Intensive effort has been expended on modifying and optimizing the cathode materials to establish efficient electron injection into the adjoining organic layer, such as using low work function metals or composites, doping cathode or organic layers, or inserting an insulating layer at the metal/organic interface.
Ultrathin lithium fluoride (LiF) with an overlaying aluminum film is a commonly used high-performance cathode, which significantly enhances electron injection by liberating the low work-function metal Li via LiF dissociation in the coexistence of Al, LiF, and tris-(8-hydroxyquinoline) aluminum (Alq3). However, no noticeable dissociation or reaction between LiF/Al occurs in the absence of Alq3, thus limiting its use for OLEDs employing other emissive organic materials. It is therefore highly desirable to provide a material which has a low work function and yet is applicable in a wide range of organic electronic/optoelectronic devices.
Furthermore, in a fundamental organic LED structure, organic layers in the OLEDs may be deposited on glass with transparent anodes (usually indium tin oxide, ITO) and capped by reflective metal cathodes. These kind of devices are hereinafter referred as bottom emitting (BE) OLEDs. However, for some applications (e.g. active matrix OLEDs), it is necessary to fabricate a surface-emitting (SE) OLED structure on opaque substrates (e.g. silicon based electronics driver and pixel switching elements). Thus, it is highly desirable to develop high performance and transparent cathodes that can be conveniently deposited on organic materials.